It is well known in the art that a voltage regulator is basically a comparator that compares a voltage reference and a voltage proportional to an output voltage from the voltage regulator to develop an error signal. The error signal is amplified by a direct current (DC) amplifier and then used to drive the output voltage to a predetermined level, thereby forming a closed loop system. To maintain a precise voltage regulation throughout a normal range of load currents which the voltage regulator must supply during operation after power-up, very high loop gain is required.
Very high loop gain also results in a momentarily high current drain from the power source supplying power to the conventional voltage regulator whenever the load condition at the output of the voltage regulator exhibits a momentarily low impedance. Such a load condition can be exhibited, for example, immediately after power is applied to a capacitive load, which can cause a momentarily heavy inrush current to bring the capacitive load up to the regulated output voltage. Some electronic devices, e.g., battery powered communication receivers, utilize a voltage regulator for powering circuit elements that require a precisely regulated voltage level during normal device operation. Other circuit elements that can tolerate less precise voltage are preferably not powered from the voltage regulator, but are powered directly from the battery, for greatest economy and energy efficiency. Such other circuit elements can include, for example, a microprocessor and a memory element.
Some batteries unfortunately have relatively high internal impedance and thus are not able to supply a load current substantially higher than the normal range of load currents, even for a brief period. The voltage of such a battery can drop momentarily to a very low level when the battery is subjected to the high current drain from the power source supplying power to the conventional voltage regulator whenever the load condition exhibits a momentarily low impedance. When the battery voltage drops to the very low level, deleterious effects can occur in the circuit elements that are powered directly from the battery. For example, the microprocessor can reset, and the memory contents can be lost.
It would be possible to use a lower loop gain in the conventional voltage regulator to limit the battery current drain by allowing the regulated output voltage to drop out of regulation under heavy load conditions. Lower loop gain, however, would destroy the ability of the voltage regulator to maintain the desirable precise voltage regulation throughout the normal range of load currents which the voltage regulator must supply during normal operation.
Thus, what is needed is a voltage regulator that can maintain a precise voltage regulation throughout a normal range of load currents which the voltage regulator must supply during operation after power-up, without allowing a high current drain from the power source under momentary heavy load conditions, such as a power-up of a capacitive load.